fix(rpc): Improve upgradetohd (#5455)

## Issue being fixed or feature implemented
Allow `upgradetohd` in IBD, better errors, no GUI lock-up

## What was done?
Pls see individual commits. Most of it is changes in whitespaces, might
want to use ?w=1 to review i.e.
https://github.com/dashpay/dash/pull/5455/files?w=1

## How Has This Been Tested?
run tests, try `upgradetohd` on testnet

## Breaking Changes
n/a

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

src/rpc/client.cpp

@@ -201,7 +201,6 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "createwallet", 1, "disable_private_keys"},
     { "createwallet", 2, "blank"},
     { "createwallet", 4, "avoid_reuse"},
-    { "upgradetohd", 3, "rescan"},
     { "createwallet", 5, "load_on_startup"},
     { "loadwallet", 1, "load_on_startup"},
     { "unloadwallet", 1, "load_on_startup"},

src/wallet/rpcwallet.cpp

@@ -2665,6 +2665,9 @@ static UniValue upgradetohd(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_WALLET_ERROR, "This type of wallet does not support this command");
     }
 
+    bool generate_mnemonic = request.params[0].isNull() || request.params[0].get_str().empty();
+
+    {
         LOCK(pwallet->cs_wallet);
 
         // Do not do anything to HD wallets
@@ -2697,14 +2700,9 @@ static UniValue upgradetohd(const JSONRPCRequest& request)
             }
         }
 
-    bool generate_mnemonic = request.params[0].isNull() || request.params[0].get_str().empty();
-
         SecureString secureMnemonic;
         secureMnemonic.reserve(256);
         if (!generate_mnemonic) {
-        if (pwallet->chain().isInitialBlockDownload()) {
-            throw JSONRPCError(RPC_CLIENT_IN_INITIAL_DOWNLOAD, "Cannot set mnemonic while still in Initial Block Download");
-        }
             secureMnemonic = request.params[0].get_str().c_str();
         }
 
@@ -2729,6 +2727,7 @@ static UniValue upgradetohd(const JSONRPCRequest& request)
                 }
             }
         }
+    }
 
     // If you are generating new mnemonic it is assumed that the addresses have never gotten a transaction before, so you don't need to rescan for transactions
     bool rescan = request.params[3].isNull() ? !generate_mnemonic : request.params[3].get_bool();
@@ -2737,7 +2736,16 @@ static UniValue upgradetohd(const JSONRPCRequest& request)
         if (!reserver.reserve()) {
             throw JSONRPCError(RPC_WALLET_ERROR, "Wallet is currently rescanning. Abort existing rescan or wait.");
         }
-        pwallet->ScanForWalletTransactions(pwallet->chain().getBlockHash(0), 0, {}, reserver, true);
+        CWallet::ScanResult result = pwallet->ScanForWalletTransactions(pwallet->chain().getBlockHash(0), 0, {}, reserver, true);
+        switch (result.status) {
+        case CWallet::ScanResult::SUCCESS:
+            break;
+        case CWallet::ScanResult::FAILURE:
+            throw JSONRPCError(RPC_MISC_ERROR, "Rescan failed. Potentially corrupted data files.");
+        case CWallet::ScanResult::USER_ABORT:
+            throw JSONRPCError(RPC_MISC_ERROR, "Rescan aborted.");
+            // no default case, so the compiler can warn about missing cases
+        }
     }
 
     return true;